Olfaction, the sense of smell, begins in the ciliary processes of olfactory receptor neurons. These odorant sensitive neurons are asymmetric bipolar cells situated in a pseudostriatified epithelium, resting on a basal layer of proliferative stem cells and supported apically by sustentacular cells. Each neuron sends a long axon inward through the cribiform plate to synapse in the olfactory bulb, and a dendrite outward into the mucosal layer. The dendrite terminates in a knob from which sprout a number of cilia, wherein occurs olfactory signal transduction.
Cyclic nucleotides, especially cAMP, appear to play a crucial role in olfactory signal transduction. Odorants stimulate adenylyl cyclase activity of olfactory cilia (Pace, et al. (1985) Nature, 316:255-258; Sklar, et al. (1986) J. Biol. Chem., 261:15538-15543; Breer, et al. (1990) Nature 345:65-68) and primary cultures of olfactory neurons (Ronnett, et al. (1991) J. Neurosci., 11:1243-1255; Ronnett, et al. (1991) Proc. Natl. Acad. Sci. USA, 88:2366-2369), effects which can occur extremely rapidly and at very low concentrations of odorants (Boekhoff, et al. (1990) EMBO Journal, 9:2453-2458; Ronnett, et al. (1991) Proc. Natl. Acad. Sci. USA, 88:2366-2369). The rapid decline in cAMP levels suggests an important regulatory role for phosphodiesterase (PDE).
Several types of PDE have been differentiated, based on their affinities for cyclic nucleotides, activation or inhibition by cGMP, and regulation by Ca.sup.2+ /calmodulin (Thompson, et al. (1972) Meth. Enzymol., 34:205-212; Kincaid, et al. (1988) Meth. Enzymol., 159:457-470; Beavo, J.A. (1988) Advances in Second Messenger and Phosphoprotein Research., 22:1-38). A Ca.sup.2+ /calmodulin PDE (CAM-PDE) activity might be relevant to olfaction, since there is evidence of a role for Ca.sup.2+ in olfaction. For instance, odorants stimulate the influx of Ca.sup.2+ into olfactory neurons (Restrepo, et al. (1990) Science 249:1166-1168), calmodulin antagonists alter the electro-olfactogram (Winegar, et al. (1988) Biochem. Physiol. 91A:309-315) and odorants potently stimulate inositol 1,4,5-trisphosphate (IP.sub.3) formation which is associated with the release of sequestered Ca.sup.2+ (Boekhoff, et al. (1990) EMBO Journal, 9:2453-2458; Wood, et al. (1990) Chem. Senses, 15:655). Moreover, the adenylyl cyclase whose activity is enhanced by odorants in primary olfactory neuronal cultures, is absolutely dependent upon Ca.sup.2+ (Ronnett, et al. (1991) Proc. Natl. Acad. Sci. USA, 88:2366-2369). There is a need in the art to determine if a PDE plays a role in olfaction, and if it does, to determine what properties this PDE displays.